1. Field of the Invention
This invention relates to a ceramic composite comprising particles of an inorganic compound and particles and whiskers of silicon nitride, silicon oxynitride and silicon oxide, and to a process for producing the ceramic composite. Further, this invention relates to an electric conductor, heater and sliding material comprising the ceramic composite.
2. Description of Related Art
Ceramics to be used as a conducting material, structural material or etc. has recently been required to have a high high-temperature strength, high toughness, high thermal shock resistance and high accuracy of dimension. As such ceramics are known inorganic compounds such as carbides, nitrides, oxides, borides, silicides and oxynitrides, for example, silicon nitride, silicon carbide, alumina and zirconia. These ceramic materials each is hardly used by itself to have so excellent properties as mentioned above. Therefore, many proposals have been made to achieve such properties by selecting the combinations of the ceramic materials and changing the design of structure.
As a heater there has hitherto been used a metal heater or ceramic heater. The metal heater includes nichrome or tantalumn. This heater has such a problem that it is poor in heat resistance and degraded at a temperature of 1,000.degree. C. or higher.
As a ceramic heater practically used there are silicon carbide, stabilized zirconia, lanthanchromite, molybdenum silicide and the like. These ceramic materials have a relatively high electric resistance, and thermo-runaway easily occurs and temperature-controlling is very hard in such heater since such ceramic materials have a negative resistance-temperature coefficient. Furthermore, these ceramic materials have such defects that they are poor in mechanical strength and thermal shock resistance. Therefore, new electrically conductive ceramics as substitutions for the above-mentioned ceramic materials have been demanded and various trials have been made.
As described in, for example, Japanese Patent KOKAI (Laid-Open) No. 50-84936, SiC or Si.sub.3 N.sub.4 is mixed with an electrically conductive compound and subjected to hot press sintering to eliminate the problem or defects as mentioned above. However, the hot press sintering method makes it possible to densify sintered bodies but the method has such a problem that cost for production is high since a great amount of energy is required for production.
As described in Japanese Patent KOKAI (Laid-Open) No. 60-44990, an electrically conductive ceramic enveloped with an electrically insulating ceramic is sintered by hot pressing, which requires a great amount of energy. Further, the hot pressing makes it impossible to mold and sinter products having a complicated shape. Generally, to integrate and sinter two or more ceramics having electric resistances different from each other, their coefficients of thermal expansion must be adjusted, but the hot pressing is carried out at so high temperatures that a small difference in coefficient of thermal expansion makes it easy for cracks to form.
Furthermore, Japanese Patent KOKAI (Laid-Open) No. 60-60983 discloses a process in which an electrically conductive material is mixed with Si.sub.3 N.sub.4 particles and the mixture is sintered by pressureless sintering. In this process, a sintering aid is used and hence softening or deformation takes place inconveniently at high temperatures, and further a volume shrinkage of about 40 to 60% is brought about on sintering to induce deformation of sintered bodies. On the other hand, the sintering of the conductive material and Si.sub.3 N.sub.4 particles in the absence of any sintering aid do not improve the density of the sintered bodies and, therefore, the specific resistance of the bodies is inconveniently high. Thus, the electrically conductive product according to this Patent KOKAI has unsatisfactory performance.
The Japanese Patent KOKAI references above do not take any account of shrinkage of molded bodies on sintering. Therefore, in these references shrinkage is brought about.
Generally, SiC, Si.sub.3 N.sub.4 or the like is known as an engineering ceramic suitable for a structural material for engines or turbines. These ceramics are excellent in heat resistance and they are sintered by pressureless sintering, pressure sintering or reaction sintering. With the pressureless sintering and pressure sintering, a rate of dimensional change before and after sintering is 15 to 20%. Thus, these sintering techniques provide deformation and poor accuracy of dimension. On the other hand, it is known that the reaction sintering technique provides a lower rate of dimensional change than that of the other sintering techniques, but nevertheless, the rate is as high as about 1 to 1.5%, as shown in Japanese Patent KOKAI (Laid-Open) No. 58-140375.
As a material having a low rate of dimensional change, there is known a bonded Si.sub.3 N.sub.4 product which has been used as a refractory material. Such material has a mechanical strength of only about 50 MN/m.sup.2, as shown in Japanese Patent KOKAI (Laid-Open) No. 58-88169, and is not suitable as a structural material.
Furthermore, Japanese Patent KOKAI (Laid-Open) No. 61-201662 which has filed by the present applicant, discloses that a fully satisfactory rate of dimensional change on sintering is obtained but a body having a complicated shape is hardly obtained due to poor flowability of a starting mixture, and further the resultant sintered body has an insufficient mechanical strength.
Some of the reasons for hardly popularizing the engineering ceramics are that the rate of dimensional change on sintering is high and sintered bodies are hardly workable due to the high hardness and brittleness thereof. It is not too much to say that there have never been any practical process for producing a ceramic material having a high strength, low rate of dimensional change and high accuracy of dimension.
On the other hand, the ceramic material has been used as a sliding material for sealing having an excellent resistance to thermal shock.
Generally, working of a machine is always accompanied with sliding. Reduction of friction and wear will induce saving of energy and long life of a sliding material. For example, a floating seal comprising O-ring 4 and sealing elements 5 and 6 as shown in FIG. 17, has been used as a sealing means for cars or construction machines. In this means, one of these sealing elements is fixed, and the other is rotated in contact with the fixed element at the sliding surface 7 thereof and set through a lubricant 8 in a floating state with respect to an axis 9, thereby keeping the sealing property of the means. Therefore, this means is most suitable for sealing or protecting from earth and sand and has widely been used. However, this floating seal has such a defect that it is poor in heat resistance and wear resistance since it is made of a cast iron material. Another sealing means is known in which one of sealing elements 10 is made of Cr cast iron and the other 11 is made by injection molding. This sealing means has also the same defect as mentioned above.
On the other hand, a ceramic-made sliding means has recently been provided. A typical example of the ceramics for the sliding means includes SiC, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3 or ZrO.sub.2, but it is practically required to have the following properties as the sliding material (see "Kogyo Zairyo", Vol. 31, No. 12, pp. 139-146, particularly 140):
(1) a high hardness and an excellent resistance to wearness and seizing;
(2) a good affinity to a sliding partner (a low coefficient of friction and a small amount of the partner worn);
(3) a good corrosion resistance;
(4) a high strength, toughness and resistance to thermal shock;
(5) a small amount of deformation by stress and/or heat at sliding surfaces to keep a good lubrication state on the sliding surfaces (i.e., a high Young's modulus and a small coefficient of thermal expansion);
(6) a high thermal conductivity (The increase of the coefficient of friction and thermal deformation on the sliding surfaces are prevented by increasing the resistance to thermal shock and inhibiting the accumulation of frictional heat.); and
(7) a low specific gravity (centrifugal stress is reduced).
These required properties all are not satisfied by known materials. For example, a metallic material is questionable in the resistance to seizing. Al.sub.2 O.sub.3 itself has a low resistance to thermal shock, and since it has a high coefficient of thermal expansion, it is easily deformed by heat on the sliding surfaces thereof. ZrO.sub.2 itself also has a low resistance to thermal shock and a high coefficient of thermal expansion. Si.sub.3 N.sub.4 itself has a low thermal conductivity. SiC alone has a high hardness and high thermal conductivity but has a poor resistance to thermal shock. A sliding material having both excellent wear resistance and corrosion resistance and conspicuously improved resistance to thermal shock has been demanded.
Among these sliding materials SiC is widely used in a sintered form. For example, Japanese Patent KOKAI (Laid-Open) No. 61-163174 discloses "a sliding material comprising an SiC sintered porous body having an excellent sliding property under wet conditions" and Japanese Patent KOKAI (Laid-Open) No. 55-100421 discloses "a sliding means comprising a fixed part and rotating part, any one of the parts being made of Si.sub.3 N.sub.4 and the other made of SiC".
However, these Japanese Patent KOKAI references all do not draw any attention to resistance to thermal shock and one-side contact.
Thus, the prior art materials have no combination of good wear resistance and thermal shock resistance and further have such drawbacks that cracks are generated due to a rapid change of temperatures and they allow their partners to be worn.